Extended Surface Heat Transfer Research Articles

Improving the performance of a triple-flow solar air collector using recyclable aluminum cans as extended heat transfer surfaces: An energetic, exergetic, economic and environmental survey

In the current work, the effect of integrating recyclable aluminum cans as tubular fins to a triple-flow solar air collector (TFSAC) has been numerically and experimentally investigated. In this regard, conventional (TFSAC) and aluminum can-integrated (TFSAC/F) systems have been simulated by applying computational fluid dynamics approach. Moreover, developed TFSACs have been fabricated and experimentally analyzed at fixed flow rate of 0.01 kg/s. According to the experimentally obtained results, utilizing recyclable aluminum cans as fins improved the average temperature difference as 14.02%. Additionally, average exergetic efficiencies for TFSAC and TFSAC/F were calculated as 18.87% and 23.25%, respectively. Economic and environmental analyses have also been performed for the developed TFSACs within the scope of the present survey. The simple payback periods for TFSAC and TFSAC/F were gained as 1.178 and 1.035 years, respectively. Moreover, the yearly CO2 savings for TFSAC and TFSAC/F analyzed in this work were attained as 0.149 and 0.169, respectively. It should be stated that mean deviation between numerically and experimentally acquired exit air temperature values are 3.71% and 7.62%, respectively for TFSAC and TFSAC/F.

Active cooling of high concentrator panels with multi-junction modules by confined liquid impingement over the extended surfaces

The use of an efficient and reliable method to cool multi-junction cells in a high concentrator photovoltaic/thermal system leads to higher net output power in addition to improving the service life of the cells. The present study numerically investigates the cooling effect of a liquid-confined submerged jet impinged over the extended heat transfer surfaces in the back of the solar cells. The method is considered for the four high concentrator photovoltaic/thermal system designs. All systems include panels with two symmetry lines and four symmetric multi-junction modules under a concentration ratio of 1000 suns and a laminar flow of coolant. Extended heat transfer surfaces were designed as arrangements of rectangular fins inside a heat sink. According to the results, the hybrid cooling designs exhibited better cooling performance than confined jet impingement cooling. The temperature of the solar cells in one of the proposed designs at an inlet mass flow rate of 200g/min was 58.7 °C, which was smaller than the corresponding temperature of 70.4 °C in a similar finless design. For the proposed design, the electrical efficiency of the cells improved from 38.16 % to 38.71 % at the same inlet mass flow rate. This cooling scheme also reduced the temperature non-uniformity across the cells to 3 °C in some of the proposed designs.

Enhancing the performance of an unglazed solar air collector using mesh tubes and Fe3O4 nano-enhanced absorber coating

In the current article, it is intended to improve the performance of an unglazed solar air collector using mesh tubes as extended heat transfer surfaces and nano-enhanced black paint as a thermal conductivity booster of the absorber coating material. In this regard, three types of unglazed solar air collectors have been designed, produced and simultaneously tested containing a conventional (unmodified) system, a system with only mesh tube modification and a system with combined usage of mesh tubes and nano-enhanced absorber coating. It should be stated that Fe3O4 nanoparticles have been utilized within the scope of this work. Integrating nanoparticles to the absorber coating material (industrial matt black paint) averagely improved the thermal conductivity as 0.031 W/mK. The experimental process was tested at fixed air flow rate (0.0115 kg/s) in winter climatic conditions. As result of the experimental analysis, average thermal efficiency values were attained between 45.11 and 63.36%. Combined usage of mesh tubes and nano-enhanced black paint upgraded the mean thermal performance as 40.45% in comparison to the unmodified system. Also, obtained exergetic efficiencies are in the range of 5.49–9.96%. In addition to the energy-exergy analysis, enviro-economic survey was performed within the scope of the current work. Payback periods of the analyzed systems were found between 0.31 and 0.34 years.

EXPERIMENTAL ANALYSIS OF A QUADRUPLE-PASS SOLAR AIR HEATER WITH EXTENDED HEAT TRANSFER SURFACES AND NANO-ENHANCED ABSORBER COATING

In recent years, selective solar absorber surface coating applications have become quite popular in terms of enhancing the thermal performance of solar-thermal systems. Generally, nano-sized particles with high thermal conductivity values are preferred to be utilized in this type of modifications. In this study, nanoparticle-enhanced absorber coating material was applied to a quadruple-pass solar air heater to improve the thermal performance. In this regard, copper oxide nanoparticles with 38-nm average particle size and 32.9 W/m K thermal conductivity were mixed with industrial matte-black paint (2% wt./wt.). The obtained mixture was then applied to the heater and the developed heater has been experimented at three flow rate values. Embedding nanoparticles to the pure paint improved the average thermal conductivity as 0.033 W/m K (from 0.6392 W/m K to 0.6722 W/m K). According to the experimentally attained outcomes, mean thermal efficiency values were obtained in the range of 71.27-79.63%. Improving the flow rate from 0.007 kg/s to 0.013 kg/s enhanced the sustainability index value from 1.1696 to 1.2123. Moreover, maximum instantaneous thermal efficiency was found to be 4.05% higher in comparison with the system without nano-enhanced modification which was tested in a previous work.

Studying transient heat transfer in a cylindrical pin fin with radiation and convection

A simple experimental fin device was used to develop a laboratory procedure for undergraduate engineering students, in order to enhance their understanding of the transfer of thermal energy. This experiment exposes the student to several important concepts, namely one-dimensional, time-dependent heat transfer in extended surfaces by conduction, convection and radiation. In a few simple steps, students have the opportunity to compare the measured to the predicted temperature profiles obtained at different times using an analytical complex solution and to calculate convective and radiative heat coefficients, such as the relative predominance of convection heat transfer with respect to radiation.

SIMPLIFIED FORMULAS FOR SOME BESSEL FUNCTIONS AND THEIR APPLICATIONS IN EXTENDED SURFACE HEAT TRANSFER

Bessel functions find many applications in Physics and Engineering fields. Some of these applications are in the analysis of extended surface heat transfer where the cross-sections vary. Tables of various kinds of Bessel functions are available in most handbooks of mathematics. However, the use of tables is not always convenient, particularly for applications where many values must be computed. In the applications of Bessel functions in extended surface heat transfer, graphs are also available to provide quick evaluations of the values needed. However, reading these graphs always needs interpolation; this will be cumbersome and time-consuming if there are many readings to be taken. Mathematical formulas for Bessel functions are available but they are usually complicated. Software to calculate values of Bessel functions is also available. Excel, Maple, and Mathematica can also be used to compute the values of Bessel functions. A user can write a program for an application that involves Bessel functions. However, the use of Bessel functions in Excel is limited while Maple and Mathematica are expensive commercial software. In this paper, formulas for Bessel functions of and are simplified with adequate accuracy that can be used to easily compute values needed in the extended surface heat transfer analysis. It is found that errors for and are relatively small (maximum errors are 0.004% and 0.003%, respectively) in the range of 0.05 to 3.75 while the maximum error for is 3.678% for the same range. However, the maximum error for is reduced to 0.166 if the range is from 0.25 to 3.75.

Entropy Analysis of the Thermal Convection of Nanosuspension within a Chamber with a Heat-Conducting Solid Fin

Heat transport augmentation in closed chambers can be achieved using nanofluids and extended heat transfer surfaces. This research is devoted to the computational analysis of natural convection energy transport and entropy emission within a closed region, with isothermal vertical borders and a heat-conducting solid fin placed on the hot border. Horizontal walls were assumed to be adiabatic. Control relations written using non-primitive variables with experimentally based correlations for nanofluid properties were computed by the finite difference technique. The impacts of the fin size, fin position, and nanoadditive concentration on energy transfer performance and entropy production were studied. It was found that location of the long fin near the bottom wall allowed for the intensification of convective heat transfer within the chamber. Moreover, this position was characterized by high entropy generation. Therefore, the minimization of the entropy generation can define the optimal location of the heat-conducting fin using the obtained results. An addition of nanoparticles reduced the heat transfer strength and minimized the entropy generation.

Numerical Simulation of Solid and Porous Fins’ Impact on Heat Transfer Performance in a Differentially Heated Chamber

The development of different industrial fields, including mechanical and power engineering and electronics, demands the augmentation of heat transfer in engineering devices. Such enhancement can be achieved by adding extended heat transfer surfaces to the heated walls or heat-generating elements. This investigation is devoted to the numerical analysis of natural convective energy transport in a differentially heated chamber with isothermal vertical walls and a fin system mounted on the heated wall. The developed in-house computational code has been comprehensively validated. The Forchheimer–Brinkman extended Darcy model has been employed for the numerical simulation of transport phenomena in a porous material. The partial differential equations written, employing non-primitive variables, have been worked out by the finite difference technique. Analysis has been performed for solid and porous fins with various fin materials, amounts and heights. It has been revealed that porous fins provide a very good technique for the intensification of energy removal from heated surfaces.

Impact of porous complicated fin and sinusoidal-heated wall on thermogravitational convection of different nanofluids in a square domain

Porous fins due to high extended heat transfer surface are widely used in different engineering devices including heat exchangers, solar collectors, and chemical reactors. Location of these fins within the working area can be very manifold. Therefore, the present research deals with computational analysis of complicated porous fin influence on nanofluid flow and heat transfer within the heated/cooled cabinet where one vertical wall is kept at non-uniform sinusoidal temperature. The considered fin consists of the mounted porous part at the bottom adiabatic wall and inner porous zone. Investigation has been performed using the boundary-value problem for the partial differential equations formed on the basis of the conservation laws using the experimentally-based correlations for thermal properties of nanofluid. Employing the finite difference techniques the useful results have been obtained illustrating an impact of the inner porous part penetration, location of this part and nanoparticles concentration. It has been ascertained that vertical position and penetration of internal porous block can be efficacious characteristics for the heat transfer augmentation. Moreover, an addition of alumina nanoparticles suppresses the heat transfer and convective flow.

Experimental and Computational Analysis of Additive Manufactured Augmented Backside Liner Cooling Surfaces for Use in Micro-Gas Turbines

Abstract Augmented backside cooling refers to the enhancement of the backside convection of a combustor liner using extended heat transfer surfaces to fully utilize the cooling air by maximizing the heat transfer to pumping ratio characteristic. Although film cooling has and still is widely used in the gas turbine industry, augmented backside cooling has been in development for decades now. The reason for this is to reduce the amount of air used for liner cooling and to also reduce the emissions caused by using film cooling in the primary zones. In the case of micro-gas turbines, emissions are of even greater importance, since the regulations for such engines will most likely become stricter in the following years due to a global effort to reduce emissions. Furthermore, the liners investigated in this paper are for a 10 kWe micro-turbine, destine for various potential markets, such as combined heat and power for houses, electric vehicle hybrids and even small unmanned aerial vehicles. The majority of these markets require long service intervals, which in turn requires the combustor liners to be under the least amount of thermal stress possible. The desire to also increase combustor inlet temperatures with the use of recuperated exhaust gases, which in turn increase the overall system efficiency, limits the cooling effectiveness of the inlet air. Due to all these reasons, an advanced form of augmented backside cooling would be of substantial significance in such a system. Currently, some very simple designs are used in the form of straight plain fins, transverse strips, or other similar geometries, but the creation of high heat transfer efficiency surfaces in such small sizes becomes very difficult with traditional subtractive manufacturing methods. When using additive manufacturing though these types of surfaces are not an issue. This paper covers the comparison of experimental results with conjugate heat transfer computational fluid dynamics (CFD) models and empirical heat balance models for two different additively manufactured (AM) liner cooling geometries and an AM blank liner. The two cooling fin geometries include a rotating plain fin and an offset strip fin. The liners were tested in an AM-built reverse flow radial swirl stabilized combustion chamber at a variety of operating conditions. During the experiments, the surfaces were compared using a thermal camera to record the outer liner temperature, which was viewed through a quartz outer casing. The experimental results showed that the cooling surfaces were effective at reducing the liner temperatures with minimal pressure losses for multiple operating points. Those results were then compared against the conjugate heat transfer CFD models and the empirical calculations used to design the surfaces initially. From this comparison, it was noticed both the CFD and empirical calculations under-predicted the wall temperatures. This is thought to be due to inaccuracies in the predicted flame temperatures and the assumed emissivity values used to calibrate the thermal imaging camera. Further uncertainties arise from the assumption of a constant air and hot gas temperature and mass flow along the cooling surfaces and the lack of data for the surface roughness of the parts.

Computational study of heat-transfer in extended surfaces with various geometries

In this work, an analysis has been done on extended surfaces with various geometries to identify the better heat transfer rate. The geometries of extended surfaces namely – bar fins, stepped fins, stepped fins with indentations have been modelled, investigated and studied. Based on the design identification and numerical analysis the extended surfaces with increases surface area has increased heat transfer. From numerical data the fins with indentations has increased surface area and heat transfer. The indentations have been designed with varying diameter used in the fins are 4mm, 3mm, 2mm respectively and analysed using a fixed computational method. It is found that the spherical notches with varying diameter were more effective than the other fins and making the notches increases the heat transfer was much effective than increasing the diameter of the indentation in the stepped fin. This work shows the design, analysis, and result of some extended surface geometries with fixed input boundary condition.

Естественная конвекция в замкнутой полости с реберной структурой при наличии неравномерного температурного профиля на вертикальной стенке

The development of energy instrumentation, electronic industry and energy in general is inextrica-bly linked with the intensification of heat and mass transfer occurring in the base units and aggre-gates of energy systems. One of the approaches to solving this problem is to create an extended heat transfer surface by introducing a rib structure or porous inserts. This work is devoted to math-ematical modeling of natural convection in an enclosure in the presence of a rib structure and non-uniform temperature profile on one of the vertical walls. The governing partial differential equa-tions written using dimensionless non-primitive variables “stream function – vorticity – tempera-ture” based on the Boussinesq approximation, in combination with initial and boundary conditions, have been worked out on the basis of the finite difference method. The developed computational model was varified using the mesh independence test and benchmark problems solved by other au-thors. Numerical investigations of unsteady natural convection of viscous fluid in a cavity with a variable temperature on the left wall under an influence of a rib structure have been carried out for the following values of governing parameters: Pr = 0.71, 103 < Ra < 106, and the number of ribs varied from one to three. Distributions of streamlines and isotherms, as well as the dependences of the average Nusselt number and average cavity temperature, were obtained for a steady mode. As a result of the analysis, it has been found that the addition of solid ribs allows to enhance the heat transfer for low Rayleigh numbers, while for Ra  105 one can find an attenuation of convective heat transfer. A growth of the fins heat conductivity characterizes the heat transfer enhancement.

Numerical evaluation of additively manufactured lattice architectures for heat sink applications

Tailoring the architectural characteristics of lattice materials at different length scales, from nano to macro, has become tenable with emerging advances in additive manufacturing. Cumulative needs of high heat dissipation rates and structural requirements along with lightweight constraints have led to the development of several heat sink fins with lattice architectures in heat exchange-applications. Here, we numerically investigate the potential of polymer-based 3D printed lattice architectures as extended heat transfer surfaces and examine the forced-convection characteristics of simple-cubic, body-centered-cubic and face-centered-cubic trusses as well as simple-cubic plate, and Kelvin and Octet periodic lattices with mesostructured architecture. All these lattices have a porosity of 77% (relative density ~23%) and surface area density in the range of 1500−2400m2/m3. Thermal and hydraulic finite element studies were conducted for fluid flow over the lattice architectures for low Reynolds number in the range of 50−360 and constant wall temperature conditions. The performance of different cell-topologies is characterized in terms of exit fluid temperature, heat transfer coefficient with respect to different reference surface areas, pressure drop per unit length, Colburn factor j, Fanning friction factor f and area goodness factor j/f. The study of the influence of thermal conductivity on heat transfer rate reveals that the polymer-based architected heat sinks perform close to their metallic counterparts when evaluated on per unit mass basis. Furthermore, body-centered-cubic truss, simple-cubic plate, and Kelvin and Octet lattice-cells were found to exhibit better thermal performance than some microchannel and open-cell foam heat sinks.

Nonstandard finite difference approximation for a generalized Fins problem

In this paper, a nonstandard finite difference scheme is employed to approximate the solution of a nonlinear second order differential equation obtained from the model of heat transfer in extended surfaces. The proposed method for the system enjoys the nonlocal approximation of nonlinear terms and requires neither any iterative procedure nor the computation of Jacobian unlike the standard finite difference method. Accuracy of the method and conditions for generating expected positive solution are also discussed in detail.

Simulation of Triplex Tube Thermal Energy Storage System Using High Temperature Phase Change Material

To increase the capacity factor of a concentrated solar thermal power plant (CSTPP) beyond the hours of sunlight, the use of thermal energy storage systems (TES) can be a promising solution. Phase change materials (PCMs) can store latent thermal energy in the course of the melting process and release it when solar energy is not available. Generally, PCMs have low thermal conductivity. One of the most commercially promising solutions is the application of an extended heat transfer surface inside the PCM container. Moreover, the distance of the heat transfer fluid (HTF) to the core of the PCM in a TES system can affect the storage performance. Accordingly, a triplex tube heat exchanger with eight fins is considered in this paper, to investigate the impact of the different velocity of HTF and different entrance pattern in a vertical PCM container. Notably, the middle enclosure of the triplex tube is filled with PCM. Numerical analysis using an enthalpy porosity technique revealed that increasing HTF velocity reduces the charging time. Also, when the HTF enters from the bottom of the container, the storage time will diminish owing to a natural convection side-effect, but if the HTF flows downward, the amount of sensible thermal energy storage is higher than in the other cases.

УЛУЧШЕНИЕ ТОПЛИВНОЙ ЭКОНОМИЧНОСТИ СУДОВОГО ВИХРЕКАМЕРНОГО ДИЗЕЛЯ УВЕЛИЧЕНИЕМ ТЕРМИЧЕСКОГО СОПРОТИВЛЕНИЯ ТЕПЛОПЕРЕДАЧИ

The article considers ship swirl-chamber diesel engines used in shipbuilding as the main and auxiliary engines. Two reasons for low profitability of the swirl chamber diesel engines are highlighted: large heat losses of the cooling working fluid due to the extended heat transfer surface of the chamber, and significant aerodynamic energy losses of compressed air during its passage through a relatively narrow channel connecting the piston chamber with the combustion chamber and the flow of gases from the swirl chamber on top the piston space. There have been proposed the methods for improving the operational performance of swirl-chamber diesels in production, in particular, their fuel efficiency. The scheme of the swirl-chamber and a section of the swirl-chamber cylinder head are presented. It has been stated that the total coefficient of thermal conductivity can be reduced if the wall of the swirl- chamber is made multi-layer. The layouts of a multi-layer cylinder-spherical wall of a swirl combustion chamber with a titanium cylinder-spherical insertion and thermal insulation of a vortex combustion chamber are given. The total thermal resistance of the spherical wall was calculated, heat loss through the multilayer spherical wall was determined, gas temperature in the vortex chamber was calculated, according to the average cycle temperature diagram. It was inferred that the amount of heat removed from the working fluid to cooling through the thermally insulated wall of the swirl-chamber will be 40% less than the amount of heat released to the cooling through the wall of the swirl-chamber of a commercial diesel engine. The difference in heat will be used to increase the indicator gas operation, which, with the same cyclic fuel supply, will lead to a decrease in the specific indicator fuel consumption, and at a constant level of internal engine losses - to a decrease in the specific effective fuel consumption.

Molecular dynamics investigation of surface roughness scale effect on interfacial thermal conductance at solid-liquid interfaces.

Non-equilibrium molecular dynamics simulations were conducted for solid-liquid-solid systems with nanometer scale grooved surfaces and an induced heat flux for a wide range of topology and solid-liquid interaction conditions to investigate the mechanism of solid-liquid heat transfer, which is the first work of such extensive detail done about the nanoscale roughness effect on heat transfer properties. Single-atom molecules were used for liquid, and the solid-liquid interaction was varied from superhydrophobic to superhydrophilic, while the groove scale was varied from single atom to several nanometers, while keeping the surface area twice that of a flat surface. Both Wenzel and Cassie wetting regimes with a clear transition point were observed due to the capillary effect inside larger grooves that were more than 5 liquid molecule diameters, while such transition was not observed at smaller scales. At the hydrophobic state, large scale grooves had lower interfacial thermal conductance (ITC) due to the Cassie regime, i.e., having unfilled grooves, while at the hydrophilic state, grooved surfaces had ITC about twice that of a flat surface, indicating an extended heat transfer surface effect regardless of the groove scale. At the superhydrophilic state, crystallization of liquid at the surface occurred, and the packing of liquid molecules had a substantial effect on ITC regardless of the groove scale. Finally, both potential energy of solid-liquid interaction and work of solid-liquid adhesion were calculated and were shown to be in similar relations to ITC for all groove scales, except for the smallest single-atom scale grooves, due to a different heat transfer mechanism.

Simulation intricacies of open-cell metal foam fin subjected to convective flow

An exact 3D digital image of 10 PPI open-cell metal foam matrix used in this study has been generated through X-ray micro-computed tomography. The precise foam geometrical model, in combination with the commercial computational software, facilitates synthesizing typical thermal applications of metal foam subjected to convective fluid flow and creates unique opportunity of probing some of the finer fluid flow aspects usually beyond the scope of most of the experimental investigations. Simulation of fluid flow through 10 PPI open porous metal foam subjected to constant terminal temperature has been carried out and validated a priori with the experimental data. The convective flow simulation, being computationally intensive, has been carried out in a small piece of metal foam to eventually extrapolate the data over the original length. The practical limitations of foam modelling leading to the deviations between experimental data and the predicted results have been explained in details. Subsequently, the temperature and flow fields have been probed to understand the complicated flow physics around the metal foam strut. This study gives a better insight of the state of the art research and development in the field of high porosity open-cell metal foam. The computational predictions have also been compared at length with the analytical outcomes based on the simple cubic model of metal foam. This work, in all, provides the end-user an overview of the associated intricacies while considering the computational, experimental and simplified models for open-cell metal foams when used as extended heat transfer surfaces or fins.

Experimental study on thermal flow characteristics in square serpentine heat exchangers mounted with louver-type turbulators

The present study aims to propose innovative louver-type turbulators to enhance the heat transfer rate in three major ways, i.e. core flow disturbance, jet impingement, and extended heat transfer surface. These louvers are installed in the twin-pass square channel with a hydraulic diameter (DH) of 45.5mm and a fully developed inlet condition. Three parameters are examined to find out the optimal design, including the pitch ratio (Pi/DH=1, 2, 3, 4, and ∞), the number of slat per half louver (1≤Ns≤4), and Reynolds number (5000≤Re≤20000). Particle Image Velocimetry (PIV) and Infrared Thermometry (IT) are respectively employed to measure the detailed velocity maps and wall temperature distributions. With acquired Nusselt number (Nu) ratio, the pressure measurements are also performed to estimate the Fanning friction factor (f) and further evaluate the thermal performance factor (TPF). The results show that both Nu‾/Nu∞ and f¯/f∞ ratios rise with descending Pi/DH and ascending Ns under the present test conditions. Among all the tested cases, the case with Pi/DH=1 and Ns=4 provides the highest Nu‾/Nu∞, almost twice the value of smooth reference; nevertheless, it suffers from high f¯/f∞ penalty. It is also found that the TPF level is a relatively weak function of Pi/DH. The new finding is that there exists a critical slat number of Ns=3 above which the TPF value is a weak function of Ns. In contrast, below the critical Ns the TPF value increases with decreasing Ns. From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi/DH and large Ns. The boundary layer disturbance, on the other hand, is more cost-effective than core flow disturbance as a mechanism to augment heat transfer from the viewpoint of thermal performance.

Application of the Thermal Fin Approximation to Modeling Voice Coil Magnetic Fields and Performance

This paper develops an approximate analytical model to describe the magnetic fluxes, forces, and armature reaction in cylindrical voice coil actuators and related electric machines. Magnetic flux is assumed to either lie in an inner region of the actuator and parallel to its axis, or in an outer region of the actuator and oriented radially. A differential reluctance model is derived for this set of assumptions, with a structure and solution set similar to those involved in extended-surface heat transfer (“fins”) and lossy transmission lines. Particular solutions for the force constant, motor constant, coil inductance, and variable-reluctance force are found for a symmetric motor with opposed axial permanent magnets on either side of an iron flux concentrator. The model results for this topology are validated using finite-element analysis and experimental measurements from a prototype.